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HS2 Chiltern Tunnel – Two More Sinkholes Revealed

HS2 Chiltern Tunnel

The location of two more sinkholes, which have formed above the HS2 tunnel in the Chilterns, is a field adjacent to Frith Hill on the outskirts of Small Heath Buckinghamshire. While these slumps were discovered five days apart, the date of finding the first feature was during routine monitoring work above the tunnel on February 7.

Also on February 12 HS2 surveyors discovered the second and larger feature in the same field.

Both sites have been fenced off. The field is privately owned and there is no public right of way across it.

Considering the effect of combination of disturbance from tunneling, and recent rainfall, weakened pre-existing weak spots which collapsed to form the sink holes, the probability of this event was possible, as HS2 said.

Following that HS2 get confident that the ground has settled,the sinkholes will be filled, whereas tunneling operations are continuing. Two sinkholes also developed above the Chiltern tunnel last year – the first at Little Missenden in May, and the second near Hyde Heath in November. Both have been repaired.

According to HS2: “All the sinkholes have occurred in areas where the tunnel is particularly shallow. Water monitoring showed no significant impact on water quality. The Chiltern hills are made of chalk bedrock which is naturally vulnerable to sinkholes. Water erodes the chalk over millions of years, creating weak spots which can collapse suddenly. Heavy rain – or disturbance linked to construction – can sometimes accelerate the process, causing weak points to collapse creating a sinkhole.”

Also a spokesperson for HS2 Ltd said: “The company had always recognised that the geology of this part of the route was particularly challenging – with poorer quality, less stable, chalk susceptible to ground movement in the final approach to the North Portal. We are working closely with the landowner and tenant; temporary fencing has been installed, and extra monitoring put in place.”

“Throughout the excavation of the Chiltern tunnels we have been engaging with all key stakeholders including Buckinghamshire Council, Affinity Water and the Environment Agency, and we will keep them updated. Safety remains our top priority,” added spokesperson.

As the longest on the project, the 16km twin-bore Chiltern tunnel runs from the South Portal near the M25 to South Heath in Buckinghamshire. Digging of this tunnel was up to two TBMs, Florence and Cecilia, which were commissioned in summer 2021.

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